Elastomer foams have excellent cushioning properties and are advantageously used for sealing materials, cushioning materials, pat materials, and the like. For example, polyester elastomer foams are used as a dustproofing material, a cushioning material, or the like for liquid crystal displays, plasma displays, organic EL displays, and the like of electric or electronic appliances such as cellular phones and digital cameras.
An aliphatic-aromatic copolyester resin foam is known as a polyester foam (refer to Patent Literatures 1 and 2). The aliphatic-aromatic copolyester resin foam is formed by extrusion foaming of a resin to which an air bubble control agent is added. However, in this forming process, addition of a large amount of the air bubble control agent in order to obtain a highly expanded structure causes curing of the resin, preventing the aliphatic-aromatic copolyester resin foam having a highly expanded structure from being obtained.
Further, a thermoplastic polyester resin foam having a density of 0.01 to 0.6 g/cm3 is known (refer to Patent Literature 3). Since the thermoplastic polyester resin foam is formed by a common molding process for carrying out foam molding of thermoplastic resins with various blowing agents, it does not have a highly expanded structure. Further, the foam has a nonuniform cell structure because it includes coarse cells and variation of the size of each cell is large.
Further, a thermoplastic polyester resin foam suitable for reduction in size, weight, and thickness for electric or electronic appliances has been known (refer to Patent Literature 4). The thermoplastic polyester resin foam is formed by foam molding a polyester elastomer composition in which powder particles are compounded as a foam nucleating agent. However, since magnesium hydroxide is used as the powder particles, only a small amount thereof can be compounded in a polyester elastomer composition, and it may be difficult to obtain a micro cell structure. This is because the polyester elastomer composition may be cured if magnesium hydroxide is compounded in a large amount.